1. Field of the Invention
The present invention relates to an apparatus for manufacturing a polysilicon thin film, and more particularly, to an apparatus for manufacturing a polysilicon thin film capable of applying a uniform electric field to a conductive thin film disposed on or under amorphous silicon to form a polysilicon thin film.
2. Description of the Related Art
Generally, amorphous silicon (a-Si) has disadvantages of low mobility and aperture ratios of electrons which are charge carriers and it is not appropriate for a CMOS process. On the contrary, in a polysilicon (poly-Si) thin film transistor (TFT), a drive circuit required for displaying image signals on pixels can be implemented on a substrate together with a pixel TFT-array, which was impossible in an amorphous silicon TFT (a-Si TFT).
Therefore, in the polysilicon thin film transistor, since there is no need for connection between a plurality of terminals and a driver IC, it is possible to increase productivity and reliability and reduce the thickness of a panel.
In addition, in the polysilicon TFT process, since silicon LSI fine machining techniques can be used as is, fine structures can be formed in interconnections, etc. As a result, since there is no pitch restriction upon TAB mounting of the driver IC of the amorphous silicon TFT, pixel size can be readily reduced and a large number of pixels can be formed within a small angle of view.
In comparison with a TFT using amorphous silicon, since a TFT using polysilicon in an active layer has a high switching ability and a channel position of the active layer is determined by self-alignment, element miniaturization and CMOS technology can be implemented. For this reason, the polysilicon TFT is used as a pixel switch device of an active matrix flat panel display, etc. (for example, a liquid crystal display or an organic light emitting diode display device), and plays an important role in practical use of chip-on-glass (COG) products having a large screen and a built-in driver. Methods for manufacturing polysilicon TFTs are classified into high-temperature manufacturing methods and low-temperature manufacturing methods. In order to form polysilicon at a high temperature, expensive materials such as quartz, etc. must be used to form a substrate, which is inappropriate for a large screen. Therefore, mass production of amorphous silicon thin films using polysilicon under low-temperature conditions is actively being researched. The low-temperature polysilicon manufacturing methods may be classified into solid phase crystallization (SPC), metal induced crystallization (MIC), metal induced lateral crystallization (MILC), excimer laser crystallization (ELC), and so on. While SPC can obtain uniform crystalline structure using an inexpensive apparatus, since high crystallization temperature and long time are needed, a substrate having a relative low thermal deformation temperature cannot be used and productivity is low. In the case of SPC, crystallization normally requires annealing of the amorphous silicon thin film for about 1 to 24 hours at a temperature of 600 to 700° C.
In addition, in the case of polysilicon manufactured through SPC, since solid phase change from an amorphous phase to a crystalline phase is accompanied by twin-growth, numerous crystal lattice defects are contained in the resultant crystal grains. These factors reduce mobility of electrons and holes of the manufactured polysilicon TFT and increase a threshold voltage. MIC has the advantage of placing amorphous silicon in contact with specific metals so that crystallization can be performed at a substantially lower temperature than the crystallization temperature of SPC.
Metals that can be used in MIC may include Ni, Pd, Ti, Al, Ag, Au, Co, Cu, Fe, Mn, etc. These metals react with amorphous silicon to form a eutectic phase or a silicide phase, promoting low-temperature crystallization. However, when MIC is applied to a process of manufacturing a polysilicon TFT, metal contamination in a channel may be severe.
MILC is an application of MIC. After forming a gate electrode instead of depositing metal on the channel, the metal is thinly deposited on a source and drain of a self-aligned structure to cause metal induced crystallization, thereby inducing lateral crystallization toward the channel.
Metals most widely used in MILC may include Ni and Pd. While polysilicon manufactured through MILC has better crystallization and higher field effect mobility than polysilicon manufactured through SPC, it may exhibit high leakage current characteristics. That is, while metal contamination is reduced in comparison with MIC, some unsolved problems remain.
Meanwhile, field aided lateral crystallization (FALC) is an improvement over MILC. While FALC is characterized by high crystallization speed and anisotropy in a crystallization direction compared to MILC, it is also unable to completely solve the contamination problem. While crystallization methods such as MIC, MILC, FALC, and so on are effective in that they reduce crystallization temperature, crystallization time is still long and crystallization is induced by all metals. Therefore, they are not free from metal contamination.
A recently developed excimer laser crystallization (ELC) method can solve the problem of metal contamination and manufacture a polysilicon thin film on a glass substrate through a low-temperature process. Since an amorphous silicon thin film deposited through low pressure chemical vapor deposition (LPCVD) or plasma enhanced chemical vapor deposition (PECVD) has a large absorption coefficient at the wavelength of an excimer laser in the infrared region (λ=308 nm), the amorphous silicon thin film can be readily melted at an appropriate energy density. When the amorphous silicon thin film is crystallized by the excimer laser, melting and solidification are performed for a very short time. From this point of view, the ELC method is not a low-temperature process in a strict sense. However, in the ELC method, since polysilicon is crystallized by rapid melting and solidification in a local region largely affected by an excimer laser, it is possible to manufacture polysilicon for a very short time (tens of nanoseconds) without damage to the substrate. That is, when a laser irradiates the amorphous silicon of a mother substrate formed of a glass substrate/insulating layer/amorphous silicon thin film for a very short time, only the amorphous silicon thin film is selectively heated to crystallize the glass substrate disposed at a lowermost layer without damage. In addition, since polysilicon generated upon phase change from liquid phase to solid phase exhibits a thermally stable crystal grain structure and remarkably reduces crystal defects in crystal grains in comparison with polysilicon generated through solid phase crystallization, polysilicon manufactured through the ELC method exhibits better characteristics than polysilicon manufactured through the other crystallization methods. Nevertheless, the ELC method has several important disadvantages, for example, a laser system problem of nonuniform laser beam irradiation, a laser process problem of a process region of an energy density for obtaining coarse crystal grains being extremely limited, and a problem of shot marks remaining in a large area. These problems lead to nonuniform crystal grain size in a polysilicon thin film constituting an active layer of a polysilicon TFT. In addition, since phase change of polysilicon from liquid phase to solid phase is accompanied by expansion of volume, a severe protrusion phenomenon occurs from a position where a grain boundary is formed to the surface. This phenomenon directly affects a gate insulating layer formed in a post process, which has adverse affects on device reliability such as breakdown voltage reduction due to unevenness of a polysilicon/gate insulating layer interface and hot carrier stress, etc. While a sequential lateral solidification (SLS) method was recently developed to overcome instability of the ELC method, and the process region of the laser energy density was successfully stabilized, shot marks and the protrusion phenomenon remain unsolved. In addition, in light of the rapid development of the flat panel display industry, there are still problems with techniques employing a laser in a crystallization process of a substrate having a size of 1 m×1 m or more which will need to be mass-produced in the near future. Moreover, since equipment for performing ELC and SLS methods is very expensive, an initial investment and maintenance costs are high.
In order to solve these problems, in Korean Patent Application No. 2007-0021252, the present inventors disclose a method of disposing a conductive thin film on or under a silicon thin film and applying an electric field to the conductive thin film to perform Joule heating, thereby achieving crystallization.
FIG. 1 is a longitudinal cross-sectional view of a polysilicon thin film manufacturing apparatus, and FIG. 2 is an enlarged view of “A” of FIG. 1.
Referring to FIGS. 1 and 2, in the conventional polysilicon thin film manufacturing apparatus 10, an amorphous silicon thin film 12 and an upper silicon dioxide substrate 13 are deposited on a lower silicon dioxide substrate 11, and a conductive thin film 14 is formed on the upper silicon dioxide substrate 13.
An electric field is applied to the silicon dioxide substrates 11 and 13 and the amorphous silicon thin film 12 through electrode terminals 15 installed at both upper ends of the conductive thin film 14, and Joule heating is performed to crystallize the amorphous silicon thin film 12.
However, during the conventional polysilicon thin film crystallization process, a large amount of heat generated by Joule heating deforms the substrate. When a power terminal cannot uniformly contact a conductive thin film due to deformation of a silicon dioxide substrate, a uniform electric field cannot be formed, which makes it difficult to form a good polysilicon thin film.